Polymer film and preparation method of polymer solution

Abstract

A polymer is swollen in a first tank. The swelling temperature is in the range of −10 to 50° C. The mixture is cooled in a first cooling vessel to the range of −100 to −10° C., and thereafter heated in a heating device to the range of 0 to 57° C. The polymer is cellulose acylate in which a degree of substitution of the acyl group for hydroxyl group is at least 2.87. A film obtained from the cellulose acylate has a negative value of ΔRe and a positive value of ΔRth, and |ΔRth| is at least 10 nm. ΔRe and ΔRth are respective difference of Re and Rth at the measurement thereof with use of wavelengths between 700 nm and 400 nm. The film is used adequately as a birefringence film, a protective film for a polarizing filter in a liquid crystal display.

1. A polymer film having an in-plane retardation Re(n) and a thickness retardation Rth(n) measured with use of a light of wavelength n (unit;nm), wherein:
 ΔRe is calculated by Re(700)−Re(400); 
 ΔRth is calculated by Rth(700)−Rth(400), a sign of said ΔRth being opposite to that of said ΔRe; and 
 the polymer comprises cellulose acylate, and the cellulose acylate has a degree of acylation of at least 2.87. 
 
     
     
       2. A polymer film as defined in  claim 1 , wherein said ΔRth satisfies a formula of 10 nm≦|ΔRth|≦1000 nm. 
     
     
       3. A polymer film as defined in  claim 1 , wherein said ΔRe is a negative value. 
     
     
       4. A polymer film as defined in  claim 1 , wherein said polymer film is used as a retardation filter in a liquid crystal display including a liquid crystal layer sandwiched between a pair of orientation layers, and said retardation filter is oppositely disposed over one of said orientation layers to said liquid crystal layer. 
     
     
       5. A polymer film as defined in  claim 1 , wherein said polymer film is used as a protective film for a polarizing filter, and said protective film is adhered to at least one surface of a polarized film of said polarizing filter. 
     
     
       6. The polymer film as defined in  claim 1 , wherein said cellulose acylate has a degree of substitution of acetyl groups in the range of 0.0 to 3.0. 
     
     
       7. The polymer film as defined in  claim 1 , wherein said cellulose acylate has a degree of substitution of acyl groups, in which the number of carbon atoms is from 3 to 22, in the range of 0.0 to 3.0. 
     
     
       8. The polymer film as defined in  claim 1 , wherein said polymer film is produced from a polymer solution which is prepared from a mixture containing a polymer and a solvent, and a method of preparing said polymer solution comprising:
 (a) regulating a temperature of said mixture in a range of −10° C. to 50° C. such that said polymer swells; 
 (b) cooling said mixture after the swelling, such that said temperature of said mixture is in the range of −100° C. to −10° C.; and 
 (c) heating said mixture after the cooling, such that said temperature of said mixture is in the range of 0° C. to 57° C. 
 
     
     
       9. The polymer film as defined in  claim 8 , wherein said solvent comprises a chlorinated solvent. 
     
     
       10. The polymer film as defined in  claim 8 , wherein said cooling (b) is performed under an increased pressure, and said method further comprises:
 (d) pressurizing and heating said mixture after said heating (c), under a pressure from 0.2 MPa to 30 MPa, such that said temperature of said mixture is in a range of 60° C. to 240° C. 
 
     
     
       11. The polymer film as defined in  claim 10 , wherein the cooling (b) is performed under an increased pressure by using a screw pump, and the pressurizing in the step of pressurizing and heating (d), is performed by using a screw pump.